Description of a new opecoelid trematode species from nototheniid fish in the Beagle Channel (sub-Antarctica).

Macvicaria magellanica n. sp. (Digenea: Opecoelidae) is the third representative of this genus occurring in fish in the Magellanic sub-region of sub-Antarctica and the first one found in sub-coastal waters in this area (the Beagle Channel). Its main taxonomic features include an elongate body, oral:ventral sucker ratio based on widths of 1.0:1.52-1.98, cirrus sac reaching to level of posterior half of the ventral sucker, testes arranged in tandem, numerous vitelline follicles divided into 2 groups separated by a gap parallel to the ventral sucker, vitelline follicles dorsally not confluent at the uterus and gonads, and egg dimensions of 40-51 × 25-32 μm. The most similar species is Macvicaria antarctica, but it differs from the newly described species in having vitelline follicles dorsally confluent at the level of the uterus and by occurring in fish associated with the Falkland-Patagonian shelf at greater depths (at the North Scotia Ridge, 300-500 m). A key to 9 Antarctic and sub-Antarctic species of Macvicaria is included.